Abstract : The feasibility of spectral characterization of a pulsed source of x rays with energies below 2 keV has been demonstrated. The method is based on calorimetric measurement of x-ray transmission through carbon foils of 1-micrometer thickness. With the use of a 12-element array of calorimeters, the complete transmitted fluence profile is obtained from a single x-ray pulse. The calorimeter elements have been designed for high sensitivity, and results show that a fluence as low as 10 microcal/sq cm is detectable. A new computer code, called EXSPEC, has been used to unfold x-ray spectra from experimental transmitted fluence profiles. (Author)